A transmission line transformer transmits electromagnetic energy by way of the traverse electromagnetic mode (TEM), or transmission line mode, instead of by way of the coupling of magnetic flux as in the case of a conventional transformer. The design and theory of various transmission line transformers are described in Sevick, J., “Transmission Line Transformers,”4th ed., Noble Publishing Corp., 2001.
FIG. 1 is a schematic illustration of a Guanella-type 1:1 transmission line transformer (TLT) 100. The TLT 100 generally includes a single, two-conductor transmission line 110 in signal communication with a two-terminal input port 112 and a two-terminal output port 114. The transmission line 110 includes a first electrical conductor 122 and a second electrical conductor 124 wound or coiled around (or threaded through) a magnetic core 126. The magnetic core 126 is typically constructed of a solid material such as ferrite or powdered iron. The TLT 100 illustrated provides an impedance transformation ratio of 1:1. That is, the output voltage and current replicate the input voltage and current. The usefulness of this type of transformer derives from the fact that the common-mode input and output potentials can differ from each other. In other words, the TLT 100 can support a longitudinal voltage drop between its input port 112 and output port 114. Although a conventional transformer also accomplishes this, the advantage of the TLT 100 is that its loss and bandwidth are greatly superior to those of a conventional transformer. These advantages are largely related to the properties of the transmission line 110 rather than the properties of the core 126.
In practice, a transmission line transformer such as shown in FIG. 1 may be constructed by winding a length of transmission line onto a ferrite or powdered iron core, or by stringing cores onto the transmission line like beads. Typical configurations of an actual transmission line include coaxial cable, twisted-pair wires, twin-lead ribbon cable, strip line, and microstrip, all of which are known to persons skilled in the art.
The Guanella-type 1:1 TLT 100 is the basic building block for more elaborate transmission-line transformer circuits. It may be employed with the input port and the output port each having one terminal grounded. Alternatively, it may be operated with both the input port and the output port floating, or balanced, with respect to ground. Alternatively, it may be operated with one of the ports floating, that is, not referenced to ground or any other point. In the latter configuration, a common use for a transmission line transformer is to convert a signal source voltage that is balanced with respect to ground to one that is referenced to ground (commonly referred to as unbalanced). A transmission line transformer utilized in this way is commonly referred to as a balun (i.e., balanced-to-unbalanced). FIGS. 2A-2D illustrate the four different configurations. Specifically, FIG. 2A illustrates both ports floating, FIG. 2B illustrates the input port unbalanced and the output port floating, FIG. 2C illustrates the input port floating and the output port unbalanced, and FIG. 2D illustrates both ports unbalanced.
The input and output impedances of a 1:1 Guanella transmission-line transformer as illustrated in FIG. 1 are equal to the impedance of the transmission line utilized to construct the transformer when each port is terminated in that same impedance. Various ways are known to connect one or more 1:1 transformers to produce composite transformers possessing impedance transformations other than 1:1, and in all of these cases the impedance of the transmission line differs from both the input and output impedances of the composite transformer.
Three prominent families of impedance-transforming transmission-line transformers are known. These three families are usually referred to as Guanella, Ruthroff, and Equal Delay, with the latter being a phase-corrected version of the Ruthroff configuration. Each of these families is capable of impedance transformations of N2 where N is any positive integer. In addition to these three main families there are various other connection schemes that can yield impedance transformations of N/M where N and M are positive integers. All of these transformers have one thing in common: the impedance of the transmission line used to construct the transformer must be equal to the square root of the transformer input impedance times the transformer output impedance. For example, to construct a transformer that transforms between 50 ohms and 200 ohms (N=2), the transmission line utilized to construct the transformer must possess a characteristic impedance of √ (50×200)=100 ohms
FIG. 3 shows a 1:9 Guanella transmission-line transformer circuit 300 that transforms between 50 ohms and 450 ohms This transformer circuit 300 utilizes three basic 1:1 transformers 302, 304, 306 with their inputs connected in parallel on the side of an input port 312 and their outputs connected in series on the side of an output port 314. The required characteristic impedance of the transmission line material used to construct this transformer circuit 300 is √ (50×450)=150 ohms Note that at the input the parallel connection of three 150-ohm transmission lines yields a net impedance of 50 ohms (150 divided by 3) while the output connection of the three lines in series gives 3×150=450 ohms In this known configuration, the three separate 1:1 transformers 302, 304, 306 are constructed on three separate cores 326, 328, 332 using the 150-ohm transmission line material, thus requiring a large footprint as compared to the basic single-core 1:1 transformer.
The two most common forms of transmission line utilized to construct transmission-line transformers are twin-lead (bonded side-by-side or twisted pair) and coaxial cable. Because coaxial cable is self-shielding it has advantages over twin-lead, especially when the transformer is required to work at both high power and at high frequencies where parasitic circuit elements can compromise performance. Unfortunately practical small-diameter coaxial cable is limited to upper impedance levels of about 100 ohms, with 18 to 75 ohms being much more common. Although high-impedance twin-lead can be readily constructed, it is physically large. Such twin-lead is typically used in large, very high power high-impedance transformers. For very small transformers, such as would be mounted on printed circuit boards (PCBs), the twin-lead is constructed from small-gauge bonded or twisted enamel-insulated magnet wire, and this is limited to impedances typically between 35 and 75 ohms, with 50 ohms being, by far, the most common.
Accordingly, there is a need for providing transmission line transformers having at least one high-impedance port without requiring the use of high-impedance transmission line material. In addition, there is a need for transmission line transformers that are DC-isolating between input ports and output ports, capable of providing broadband center-tap connection points at both input and output ports, and capable of operating with either or both ports floating or unbalanced while requiring only a single core for construction.